Polymer electrolyte membrane fuel cells (PEMFC) are expected to provide higher efficiencies, fewer environmental pollutants, and reduced operating and maintenance costs than traditional power sources. An important component of a PEMFC is a polymer electrolyte membrane (PEM). The range of potential candidates for use as membrane materials in PEMFCs is limited by a number of requirements, including chemical, thermal, and mechanical stability, high ionic conductivity, and low reactant permeability. Developments have been made in the use of sulfonic acid functionalized polymers, including membranes such as Nafion® perfluorosulfonic acid membranes.
Known membranes made from sulfonic acid functionalized polymers have been found to have inadequate performance at temperatures greater than 100° C. due, in part, to the dependence of the membranes on water for proton conduction. Above 100° C., pressure constraints limit the amount of water that can be used to hydrate a membrane. At relatively low levels of humidity, insufficient water is present within the membrane to support the transport of protons. In addition to improved performance at higher temperatures, it is also desirable to have improved mechanical stability at such temperatures.
Alternatives to perfluorosulfonic acid membranes include membranes based on aromatic engineering polymers. For example, poly(arylene ether)s, poly(arylene ether ketone)s, and poly(arylene ether sulfone)s are engineering polymers known for their chemical, thermal, and mechanical stability. Poly(arylene ether)s, poly(arylene ether ketone)s, and poly(arylene ether sulfone)s can be sulfonated to produce sulfonic-acid functionalized aromatic polymers. However, due to relatively poor control inherent in the process, post-polymerization sulfonation can result in sulfonation on the most electron-rich aromatic rings, essentially those substituted with just the ether functional groups, which are also the most activated due to subsequent thermal decomposition of the sulfonic acid groups.
Another method for producing sulfonic-acid functionalized aromatic polymers is by polymerizing sulfonated monomers, as disclosed, for example, by F. Wang et al., “Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes”, Journal of Membrane Science, Vol. 197 (1-2), pp. 231-242 (2002). This allows the sulfonic acid groups to be located on the most electron-deficient aromatic rings to improve their thermal stability. However, the proton conductivity of sulfonated aromatic polymers made by either of the two methods discussed hereinabove is limited by the acid strength of the aromatic sulfonic acid groups, especially at low relative humidity.
The use of fluorosulfonimide functional groups instead of sulfonic acid groups at similar equivalent weights can increase the proton conductivity of the resulting aromatic polymers because fluorosulfonimides possess higher acid strengths. M. Hofmann (U.S. Pat. No. 7,135,537) prepared aromatic polymers containing fluorosulfonimide functionalities in the backbone. However, all the polymers prepared also contained an ether functionality in the aromatic backbone, which decreases their stability. The higher acid strength of the fluorosulfonimide groups leads to thermal and chemical instability in the ether groups relative to comparable sulfonated aromatic polymers, and the flexibility of the ether groups increases the potential for excessive water uptake, which reduces their mechanical stability. In addition, electron-rich aromatic rings substituted with ether groups are more susceptible to chemical degradation under the oxidative conditions inherent in PEMFC, which are due, in part, to high permeability to the fuel cell reactants.
A need remains for polymers suitable for use in conductive membranes for applications such as fuel cells that exhibit good ionic conductivity, hydration, chemical, thermal, and mechanical stability at high temperatures, and low reactant permeability.